Embodiments of the invention relate to turbomachine manufacturing. More specifically, the subject matter disclosed herein relates to methods for manufacturing turbomachine impellers, e.g. impellers for centrifugal or centripetal turbomachines, such as centrifugal pumps and compressors and centripetal turbines.
Turbomachines comprise one or more rotating components and one or more stationary components. In turbo-compressors, a fluid flowing through the turbomachine is accelerated by the rotary motion of the rotating components and is slowed down in stationary components, where kinetic energy of the fluid is converted into pressure energy. In turbines or expanders, the energy contained in the fluid flowing through the turbomachine is converted into mechanical power available on the turbine shaft or on the expander shaft.
Centrifugal turbomachines, such as centrifugal pumps and compressors, are usually comprised of one or more impellers mounted for rotation in a casing. The impellers can be mounted on a rotary shaft which is supported in the casing by suitable bearings and rotates therein together with the impellers. Each impeller is comprised of a disc having a front surface and a back surface and provided with a central hole for the rotary shaft. In some known embodiments the impellers are provided with frontal teeth torsionally engaging mutually adjacent impellers to one another. A central rod axially locks the impellers together.
Blades extend from the front surface of the disc and define fluid passages therebetween, also called impeller vanes. The impeller can further comprise a shroud, which is connected to the blades and closes the fluid passages or vanes on the side opposite the disc.
The vanes or fluid passages formed between adjacent blades have usually a complex shape, which is dictated by fluid-dynamic considerations. Each impeller is combined with a diffuser, which receives fluid accelerated from the impeller and wherein kinetic energy of the fluid is converted into pressure energy, thus boosting the fluid pressure. The diffuser is usually formed in a so called stationary diaphragm housed in the machine casing.
Turbomachine components, which interact with the fluid flow, have often a rather complex shape. This applies both to the stationary components, as well as to the rotating components, such as the impellers.
Manufacturing of complex turbomachine components, such as diffusers and impellers, is sometimes performed by numerically controlled chip removal machining. However, machining by chip removal is limited to some shapes of diffuser and impeller, since not every fluid-dynamic surface can be machined by a chip removal tool. According to other known methods, impellers are manufactured by welding single components to one another. According to yet further known methods, investment casting is used for impeller manufacturing.
Another option for the manufacturing of turbomachine impellers is by so-called electric discharge machining (EDM), a technique wherein a workpiece in the shape of a disc is placed in a dielectric liquid and co-acts with an electrode. A voltage difference between the workpiece and the electrode is applied, thus generating electric sparks, which erode the surface of the workpiece. The electrode is shaped so that the required cavity is obtained by erosion. Different tools with different electrodes are used in sequence to machine the workpiece until the final impeller shape is obtained.
Also EDM has limitations and drawbacks, in particular due to the need of using several electrodes of different shapes in order to achieve the required final shape of the impeller. The electrodes are subject to wear and must often be replaced. Electric discharge machining is, moreover, a rather slow process.
According to a further known technique, turbomachine impellers can be manufactured by powder metallurgy using a hot isostatic pressing process. FIGS. 1 and 2 schematically illustrate an exemplary embodiment of this known powder metallurgy process for impeller manufacturing. FIG. 1 illustrates a schematic partial cross section view of a compressor impeller manufactured by powder metallurgy process and FIG. 2 illustrates an axonometric view of a core used for manufacturing the impeller schematically shown in FIG. 1.
Referring to FIG. 1, an impeller 100 comprises a disc 103 and a shroud 105. Between disc 103 and shroud 105 impeller blades 107 are arranged. Flow passages or vanes 109 are formed between consecutive adjacent impeller blades 107. Each flow passage 109 is bounded by impeller disc 103 and impeller shroud 105. The impeller disc 103 also forms a hub portion 103H with a hole 111, where through a shaft (not shown) passes, whereon the impeller 100 is mounted for rotation in the turbomachine casing (not shown). In FIG. 1 the solid portions of impeller 100 are hatched in different ways to show portions of the impeller which are manufactured with different materials and in different steps of a manufacturing process. The outer portions of both the impeller disc 103 and the impeller shroud 105 are manufactured separately and assembled with a core 113 arranged therebetween. The core 113 is made of a metal which can be dissolved by a suitable acid after the impeller has been manufactured. The core 113 reproduces the shape of the flow passages or vanes 109. More specifically, the core 113 has a plurality of radially extending core portions 113P, each corresponding to the empty volume of a respective flow passage or vane 109. The core portions 113P are separated by slots 115, the shape whereof corresponds to the shape of the impeller blades 107.
The impeller 100 is manufactured by placing the core 113 between two disc-shaped components, which form the outer skin of the impeller disc 103 and the outer skin of the impeller shroud 105. The cross-sectional shape of the core 113, as well as the cross-sectional shape of the outer skin portions of the impeller disc 103 and of the impeller shroud 105 are such that an empty volume is formed between the core portions 113P and both skin portions of the impeller disc 103 and the impeller shroud 105. The empty volume is then filled with a metal powder and the cavities are sealed. The unit thus obtained is subject to hot isostatic pressing, sometimes also called “HIP” or “hipping”, whereby heat and pressure are applied to the outer surfaces of the semi-finished article formed by the core, surrounding powder filling the inner cavities and the outer skin portions of the impeller disc 103 and impeller shroud 105. The metal powder is densified and solidified, thus forming the inner solid portions of the impeller 100. In order to form the flow passages or vanes 109, the core 113 must be removed. This is performed by acid etching.
The above described method has several drawbacks. The core 113 is an expensive and complex component, which must be machined from a solid workpiece. Manufacturing of the core 113 is an expensive process. The resulting core 113 is used only once for manufacturing a single impeller 100 and is subsequently destroyed.
Moreover, the above summarized technology sets heavy limitations on the choice of the materials that can be used for manufacturing the impeller. More specifically, both the metal forming the skin portions as well as the metal powder must resist the action of the acid used for removing the core. In turn, the core 113 must be manufactured with a metal which is capable of withstanding the high pressure and temperature conditions during hipping, but which is at the same time suitable for removal by acid etching.
There is therefore a need for an improved method of manufacturing complex turbomachine components such as in particular, but not limited to, centrifugal impellers for pumps and compressors.